Persistent activity of epidermal growth factor receptor (EGFR) and its family members is a common cause for cancer growth and drug resistance. In a series of bioinformatics-guided siRNA library screening experiments we identified a network of resistance-influencing proteins that included SC4MOL (sterol C4-methyl oxidase-like), a little-studied intermediate enzyme in the sterol biosynthesis pathway (Astsaturov, 2010). Our work is the first to study SC4MOL and NSDHL in cancer (Sukhanova, 2013), and our establishment of reciprocal interactions between EGFR signaling and sterol metabolism in cancer are intended to provide new opportunities to improve the efficacy of targeted therapies. We have found that silencing of SC4MOL or NSDHL, or direct addition of meiosis activating sterols (MAS), a substrate for these enzymes, markedly sensitizes cancer cells to EGFR inhibitors (Sukhanova, 2013). We have also found that arrest of the sterol pathway at the level SC4MOL or NSDHL activates the liver X receptor (LXR), which induces the expression of cholesterol efflux proteins (the ABC transporters ABCA1 and ABCG1), depletes cellular cholesterol, and reduces expression of LDL lipoprotein receptors (LDLR). Excitingly, we found that loss of SC4MOL or NSDHL was profoundly synergistic with the anti-EGFR antibody cetuximab against human xenografts, and suppressed growth of KRAS-driven tumors in a mouse genetic model. Our central hypothesis is that MAS sterol metabolites accumulate as the result of SC4MOL or NSDHL deficiency and negatively regulate cell growth by activating LXR, disrupting cholesterol uptake and biosynthesis, and suppressing oncogenic EGFR-KRAS signaling. We will address this hypothesis by performing the following specific Aims: In Aim 1, we will determine the mechanism by which metabolic substrates of SC4MOL and NSDHL regulate cholesterol homeostasis and contribute to the sensitivity of cancer cells to agents inhibiting EGFR. In Aim 2, we will use a genetic mouse model of NSDHL deficiency to determine how SC4MOL, NSDHL, and their substrates regulate the growth of normal cells versus EGFR-dependent cancer cells. In Aim 3, we will test the linked hypotheses that accelerated cholesterol metabolism defines cancers' aggressiveness and refractoriness to EGFR-targeting therapies, and that combined targeting of EGFR signaling and cholesterol metabolism via LXR will be synergistic. This project will illuminate an entirely new mechanism for regulating EGFR function. Optimally, this work will justify the idea that targeting SC4MOL, NSDHL or additional new targets downstream of MAS sterols would be beneficial for a broad spectrum of EGFR-positive human carcinomas including head and neck and pancreatic cancers.